Multi-band superfluidity and BEC-BCS crossover in novel ultrathin materials

This thesis presents an inquiry into multi-component electron-hole superfluidity in coupled ultrathin layer systems. Multi-component superfluidity is a novel quantum phenomenon that arises in semiconductors when multiple bands provide multiple pairing channels. The thesis focuses on two systems that define two very different classes of multi-band systems. We find that they can generate multi-component superfluids with fundamentally different properties. One system consists of two parallel bilayer graphene sheets and the other system is a heterostructure of two Transition Metal Dichalcogenide monolayers MoSe2 and WSe2. In the Double Bilayer Graphene system, superfluidity is multi-component because both conduction and valence band participate in the pairing. This is due to the small tunable band gap between the conduction and valence bands in bilayer graphene. This system is a novel multi-band system because, in contrast with conventional multi-band superconductors, here the multi-bands are not nested and there is a unique Fermi surface. In the double TMD monolayer system, superfluidity is multi-component because of the splitting of the bands caused by strong spin-orbit coupling. This superfluid has the same concentric subbands as in the multi-band superconductors, implying the potential existence of all the associated novel quantum phenomena that can arise from interference between the multi-condensates. The investigation of the pairing processes is carried out using a mean field multi-component approach. The Coulomb pairing interaction between electrons and holes is a long-range interaction and screening effects must be fully accounted for in our approach. We show that it is readily possible to tune these systems between the strongly interacting regime and the weakly interacting regime by tuning the carrier density. Because of the different pairing symmetries, the two-component condensates in these two systems are strikingly different. In Double Bilayer Graphene, the two condensates are strongly coupled. However, the closeness of the valence band to the conduction band contributes in a strong way to the screening and this results in a weakening of the superfluidity. On the other hand, in double TMD monolayers, the spin-orbit coupling makes the two condensates decoupled, and the large band gap makes the screening from the valence band negligible. We show that in both systems the multi-component nature of the superfluidity can be switched on and off. In Double Bilayer Graphene, by tuning the band gap, it is possible to continuously tune the proximity in energy of the conduction and valence bands and thus the importance of the valence band in the pairing and screening. In double TMD monolayers, by tuning the density and by choosing the doping in the monolayers, it is possible to activate or suppress the second component of the superfluidity. This thesis provides new insights into the field of the multi-component superfluidity: we predict observable superfluidity in two systems confirmed with recent experimental observations and we demonstrate the possibility to tune the multi-component character in both the class of materials. As a further result, we determine optimal electronic conditions and optimal ranges of carrier densities for these systems for maximising the transition temperature of the superfluidity.

Publication year:2020

Keywords:Doctoral thesis

Accessibility:Open